Thermoplastic substrates such as polycarbonates are generally characterized by advantageous properties which include clarity, high ductility, high heat deflection temperature, and dimensional stability. Many of these materials are transparent and some are suitable as replacements for glass in automotive, aerospace, and architectural applications. However, the resins often are susceptible to degradation by ultraviolet light evidenced by development of discoloration. This results in loss of transparency, mechanical degradation of the substrate and erosion of the substrate surface.
As thermoplastic substrates, such as polycarbonate, are applied to outdoor uses it becomes important to enhance ultraviolet radiation stability of the substrate. This is accomplished by treating the substrate surface with a weather resistant coating material, which contains ultraviolet light absorbing agents. Weather resistant coating systems can be prepared by incorporation of ultraviolet light absorbers, such as benzotriazoles and benzophenones, and hindered amine light stabilizers.
However, the ultraviolet light absorbing compounds, often referred to as UV absorbers, decompose or volatalize upon exposure to ultraviolet light thus losing the ability to protect and often causing loss of necessary properties. Prolonged exposure to sunlight, moisture and thermal cycling conditions can cause yellowing, delamination and formation of microcracks in the coating material, decreasing transparency. This leads to a degradation of the favorable properties of the thermoplastic substrate which the UV absorbers are originally employed to protect. Accordingly, there is an ongoing need to develop new and efficient UV absorbing compounds.
Certain 4,6-dibenzoylresorcinols (DBRs) have been found to be effective UV absorbers for haloethylene polymers. DBRs, as bulk additives, are described in U.S. Pat. Nos. 2,794,052 and 2,933,553. However, a drawback in current compositions of DBRs is their relatively low molecular weight, i.e., molecular weight less than or equal to about 400 daltons, which leads to volatility during processing of the thermoplastic substrates or cure of the coatings. This can result in contamination of the processing equipment. Efforts to increase the molecular weight of DBR by adding substituents can result in diluting the effectiveness of the UV absorber. There is a need for UV absorbers characterized by photostability and effectiveness of the DBR chromophore with low volatility.